Method for producing a membrane used to operate fuel cells and electrolyzers

ABSTRACT

Membranes for use in polymer electrolyte fuel cells or electrolyzers comprise a sulfonated aromatic polyether ether ketone of the formula (I)wherein the ion exchange equivalent (I.E.C.) of the sulfonated polyether ether ketone is in the range from 1.35 to 1.95 mmol (-SO3H)/g (polymer) and the membrane has a long-term stability of at least 1000 hours at an operating voltage of from 0.4 to 1.1 V.

The invention relates to membranes comprising sulfonated polyether etherketones (sPEEK) which, owing to a particular combination of variousparameters, are particularly useful for use in fuel fells orelectrolyzers.

Perfluorinated or partially fluorinated polymers bearing sulfonic acidgroups are sufficiently well known from the literature. Membranes whichcomprise these polymers and are suitable for electrochemical purposesshould have good membrane stabilities, sufficient chemical stabilityunder the operating conditions of fuel cells and electrolyzers and havehigh proton conductivities (A.E. Steck in Materials For Fuel CellSystems 1, Proc. Int. Symp. On New Materials for Fuel Cell Systems, O.Savadogo, P.R. Roberge, T.N. Veziroglu, Montreal 1995, pp. 74-94).However, membranes comprising these polymers are, owing to thefluorination steps necessary for the monomers, expensive and, inaddition, are difficult to process. As a result, thin membranes (<50 μm)of fluorinated materials cannot be produced or can only be produced withgreat difficulty, as a result of which water management in thesemembranes is made more difficult.

Recycling of the polymers is made difficult or even impossible by thedifficult handling of these materials, in particular by their sparingsolubility.

The preparation of sulfonated polyether ether ketones is described, forexample, in EP-A-0 008895 and EP-A-0 575 807 and also in Polymer, Vol.35,1994, pages 5491-5497.

The use of polyether ketones in fuel cells is described, for example, inWO 96/29359. Specific information as to which of the polyether etherketones described are usable under fuel cell conditions and thus ofeconomic interest is, however, not given in the prior art.

Furthermore, the usability of non-perfluorinated materials is frequentlystill disputed in the current literature. In the past, the operatingtimes which could be achieved using such materials in fuel cells werenot more than 600 hours (A. E. Steck in “New Materials For Fuel CellSystems 1”, Proc. of the 1st Intern. Symp. On New Materials For FuelCell Systems, Montreal 1995, p. 82).

It is therefore an object of the present invention to provide membranescomprising sulfonated polyether ether ketones which are particularlysuitable for use in fuel cells due to their chemical and physicalproperties and their high long-term stability. Furthermore, themembranes of the invention are an inexpensive and environmentallyfriendly substitute for membranes comprising fluorinated materials.

The present invention accordingly provides membranes which are, inparticular, suitable for use in polymer electrolyte fuel cells orelectrolyzers and comprise a sulfonated aromatic polyether ether ketoneof the formula (I)

wherein the ion exchange equivalent (I.E.C.) of the sulfonated polyetherether ketone is in the range from 1.35 to 1.95 mmol (—SO₃H)/g (polymer),preferably in the range from 1.50 to 1.75 mmol (—SO₃H)/g (polymer), andthe membrane has a long-term stability of at least 1000 hours at anoperating voltage of from 0.4 V to 1.1 V.

It has surprisingly been found that various chemical and physicalparameters such as the molecular weight or the degree of sulfonationhave to be kept within very narrow limits for sulfonated polyetherketones which are to be suitable for use in electrochemical cells suchas fuel cells or electrolysis cells.

An important parameter is the molecular weight of the polymer used. Thesulfonation of the base polymer and the associated conversion into acharge-bearing polyelectrolyte results in partial disentangling (cf. BVollmert, Molecular Heterogeneties in Polymers and Association ofMacromolecules, IUPAC Symposium Marienbad, Pure and Appl. Chem. 43,183-205, 1975-and M. Hoffmann, Die Verhakung von Fadenmolekülen und ihrEinfluB auf die Eigenschaften von Polymeren, Prog. Colloid. Pol. Sci.66, 73-86, 1979) of the polymer by mutual repulsion of the chargecenters on the polymer backbone.

The membranes of the invention comprise sulfonated polymers having amolecular weight Mw in the range from 50,000 g/mol to 310,000 g/mol,preferably from 100,000 g/mol to 240,000 g/mol (determined in NMP(N-methylpyrrolidone), 0.05% lithium chloride addition, 60° C., PScalibration, Waters column by GPC). Molecular weights which are too loware reflected in unsatisfactory mechanical properties of the membranes;molecular weights which are too high require high dilutions in thesulfonation in order to keep the viscosity within a suitable range. Highdilutions are uneconomical because of the increased consumption ofsulfuric acid (see also Comparative Example with M_(w)=390,000, Table2). In the case of polymers whose molecular weights are too high, theconcentration has to be drastically reduced prior to the sulfonationsince otherwise the solutions cannot be processed further.

The polymers used for producing the membranes of the invention have amodulus of elasticity (E modulus) in the dry state of greater than orequal to 1300 N/mm² and an elongation at break in the dry state afterstorage for four hours in a controlled atmosphere cabinet at 23° C. and50% relative atmospheric humidity of ≧20% (thickness 40 μm), preferably≧70%, in particular up to 150%. Owing to the high E modulus in the drystate, the membranes of the invention have a sufficient elongation atbreak, which is an important criterion for good further processibility.

In the wet state, the E modulus of the membranes must not drop below 100N/mm² in order to ensure, even in the moistened state, a minimumstrength of the membrane or membrane electrode unit.

A further important criterion which has to be met in order to obtainparticularly high-performance membranes according to the invention isthe degree of sulfonation of the polymers. For the purposes of thepresent invention, the degree of sulfonation is the proportion ofsulfonated repeating units as a fraction of the total number ofrepeating units. The ion exchange equivalent (I.E.C.), which isexpressed in millimol of sulfonic acid groups per gram of polymer, isproportional to this value. The reciprocal of the I.E.C. is referred toas the equivalent weight and is usually reported in gram of polymer permole of sulfonic acid groups. The I.E.C. is calculated from the ratio ofcarbon to sulfur determined by elemental analysis.

Polyether ether ketones which are suitable for the membranes of theinvention have an ion exchange equivalent of the sulfonated polyetherketone in the range from 1.35 to 1.95, in particular from 1.50 to 1.75mmol (—SO₃H)/g (polymer).

If the I.E.C. value is higher, many problems can result. At a degree ofsulfonation only slightly above the optimum degree of sulfonation,considerable swelling of the membrane on contact with water has to beexpected. This swelling behavior has a severe adverse effect on themembrane-electrode composite (see above regarding strength in the wetstate). If the degree of sulfonation is above the upper limit indicated,the polymer synthesized is not sufficiently mechanically stable incontact with water, or may even be completely or partially soluble inwater, particularly at temperatures above 50° C., which is alsoreflected in an E modulus of less than 100 N/mm^(2.)

However, the most important parameter for a proton-conducting membrane,namely the proton conductivity, increases continuously with increasingdegree of sulfonation, which is reflected in a higher power (W/cm²) of arelatively highly sulfonated membrane. It is therefore particularlydifficult to find a good balance between a very high proton conductivityand a degree of sulfonation which is as high as possible without thepolymer obtained having (in the presence of water) an excessively highsolubility and an unacceptably low mechanical strength.

Even an I.E.C. of 1.30 is reflected in a very low performance of thefuel cell (see first example in Table 1).

The sulfonated polymers used for the membranes of the invention have,measured in contact with pure water, a proton conductivity at roomtemperature of >3×10⁻³ S/cm, preferably >2×10⁻² S/cm, in particular upto 300 mS/cm.

The membranes of the invention comprising sulfonated aromatic polyetherketones of the formula (I) enable operating times of at least 1000hours, in particular ≧3000 hours, preferably ≧4000 hours, to be achievedwithout problems even using a non-perfluorinated material.

EXAMPLES

1) Preparation of the sulfonated polymer

30 g of dried polyether ketone are introduced into 420 g of concentratedsulfuric acid at about 5° C. while stirring vigorously by means of atoothed disk. The mixture is then stirred for another 30 minutes and thetemperature is subsequently increased to 50° C. over a period of 45minutes.

As soon as the desired degree of sulfonation has been reached, thesolution is cooled back down to 5° C. and is slowly poured into icewater. The product is washed with deionized water until free of sulfate(test with BaCI₂ solution), dried in a vacuum drying oven and milled.The degree of sulfonation is calculated from the carborn/sulfur ratioobtained by elemental analysis.

2) Production of the films

The milled, dry polymer (particle size about 80 μm, water content <0.5%)is quickly introduced into the appropriate amount of NMP and dissolvedunder inert gas at 80° C. with intensive stirring so as to give an about18% strength solution.

The still hot solution is filtered through a polypropylene nonwovenhaving a mean mesh opening of 1 μm and, still on the same day, is spreadon glass plates using a doctor blade and dried overnight at 80° C. underatmospheric pressure in a dust-free convection oven. The films arepeeled dry from the glass plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 are graphs of current density versus time and cell voltageversus time for an sPEEK membrane with the operating times of 1000,2000, 3000, 4000 and 5000 hours, respectively.

Lifetime test of an sPEEK membrane having a degree of sulfonation of 50%and a thickness of 40 μm over 4300 hours of operation using H₂/O₂ at 50°C., atmospheric pressure.

The power drop at 2700 hours and 3330 hours is due to the gas supplyfailing twice during the experiment. After the gas supply had beenrestored, the fuel cell generated the same power as before.

Table 1: Power data for sPEEK

The performance of the polyether ketones listed was measured using afuel cell (operating conditions: cell temperature 45° C., atmosphericpressure to max. 0.2 bar gauge pressure, moistening on the air side,electrode produced in-house containing 0.2-0.3 mg of Pt/cm²).

IEC (mmol of SO₃H/g Degree of Power Membrane polymer) sulfonation at 0.7V max. power sPEEK 1.30 42%  34 mW  52 mW at 510 mV sPEEK 1.47 50% 222mW 386 mW at 519 mV sPEEK 1.62 54% 290 mW 560 mW at 550 mV sPEEK 1.7358% 278 mW 523 mW at 523 mV sPEEK 1.80 61% 235 mW 389 mW at 490 mV sPEEK1.82 63% 229 mW 342 mW at 517 mV

Table 2:

Tear strengths, E modulus of a dry film (at 23° C., 50% atmospherichumidity) and associated molecular weights by PC in NMP

Molecular Elon- Molecular weight Degree of E modulus gation weightdistribution Membrane sulfonation [N/mm] at break M_(w) M_(w)/M_(n)sPEEK 42% (1.30) 1519 22% 154,000 2.90 sPEEK 50% (1.47) 1606 61% n.f.n.f. sPEEK 54% (1.62) 1527 59% 176,000 2.20 sPEEK 58% (1.73) 1385 100%203,000 2.94 sPEEK 61% (1.80) 713 112% 390,000 5.40

Table 3:

Proton conductivity data and mechanical properties are measured in waterat 23° C. (proton conductivity measured using a 4-pole arrangement at afrequency in the range from 30 to 3000 Hz, phase from −1 to +1 Hz). Themolecular weight data are as shown in Table 2).

Pretreatment of the membrane for the measurement of proton conductivity:place in 5% strength nitric acid for 30 minutes at 40° C. and then washwith distilled water.

Pretreatment of the membrane for measurement of the mechanicalproperties: place in 5% strength nitric acid for 30 minutes at 40° C.and then wash with distilled water. Dry at 23° C. and 50% relativeatmospheric humidity and irrigate for 30 minutes at 23° C.

Degree of Proton sulfonation E modulus Elongation at conductivityMembrane (IEC) [N/mm] break [mS/cm] sPEEK 42% (1.30) 730 107% 15 sPEEK50% (1.47) n.f. n.f. 42 sPEEK 54% (1.62) 523 211% n.f. sPEEK 58% (1.73)516 218% 57 sPEEK 61% (1.80) 180 281% 56 n.f. = no figures available

What is claimed is:
 1. A membrane comprising a sulfonated aromaticpolyether ether ketone of the formula (I)

wherein the ion exchange equivalent (I.E.C.) of the sulfonated polyetherether ketone is in the range from 1.35 to 1.95 mmol (—SO₃H)/g (polymer)and the membrane has a long-term stability of at least 1000 hours at anoperating voltage of from 0.4 to 1.1 V.
 2. A membrane as claimed inclaim 1, wherein the molecular weight M_(W) of the sulfonated polymer ofthe formula (I) is in the range from 50,000 to 310,000 /mol determinedby GPC:NMP, 0.05% LiCl addition, 60° C.
 3. A membrane as claimed inclaim 2, wherein the elongation at break of the polymer of the formula(I) in the dry state after storage for four hours in a controlledatmosphere cabinet at 23° C. and 50% relative atmospheric humidity is≧20% and the E-modulus of the polymer of the formula (I) in the drystate is >1300 N/mm.
 4. A membrane as claimed in claim 3, wherein themolecular weight M_(W) of the sulfonated polymer of the formula (I) isin the range from 100,000 g/mol to 240,000 g/mol determined inN-methylpyrrolidone by GPC at 60° C.
 5. The membrane as claimed in claim4, wherein the sulfonated polymer of the formula (I) has, in contactwith pure water, a proton conductivity >2×10⁻³ S/cm.
 6. The membrane asclaimed in claim 5, wherein the proton conductivity is up to 300 mS/cm.7. A membrane as claimed in claim 1 , wherein the sulfonated polymer ofthe formula (I) has, in contact with pure water, a proton conductivityof >3×10⁻³ S/cm.
 8. A membrane as claimed in claim 1, wherein the Emodulus of the polymer of the formula (I) in the dry state is >1300N/mm.
 9. A membrane as claimed in claim 1, wherein the elongation atbreak of the polymer of the formula (I) in the dry state after storagefor four hours in a controlled atmosphere cabinet at 23° C. and 50%relative atmospheric humidity is ≧20%.
 10. The membrane as claimed inclaim 1, wherein the ionic exchange equivalent of the sulfonatedpolyether ketone is in the range from 1.50 to 1.75 mmol (—SO₃H)/g(polymer).
 11. The membrane as claimed in claim 10, wherein theelongation at break of a polymer of the formula (I) in the dry stateafter storage for four hours in a controlled atmosphere cabinet or 23°C. and 50% relative atmospheric humidity is ≧70%.
 12. The membrane asclaimed in claim 11, wherein the elongate at break of a polymer of theformula (I) in the dry state is up to 150% therein.
 13. A fuel cell orelectrolysis cell which comprises the membrane as claimed in claim 1.